Science · Class 7 · Chapter 4

The World of Metals and Non-metals

• Metallic lustre — The shiny appearance of metals.
• Malleability — Being beaten into thin sheets.
• Ductility — Being drawn into wires.
• Sonority — Producing a ringing sound when struck.
• Rust / Rusting — The brown deposit on iron / how it forms.
• Corrosion — Gradual wearing away of a metal surface.
• Galvanisation — Coating iron with zinc to prevent rusting.
• Alloy — A mixture of metals, or metal with non-metal.

Chapter 4 · Concept 1 of 11

Properties of Metals

• Lustre — A shiny surface — copper, aluminium, iron.
• Malleability — Can be beaten into thin sheets — gold and silver most of all.
• Ductility — Can be drawn into wires — 1 g of gold stretches into 2 km of wire.
• Sonority — Produces a ringing sound when struck — think bells.

Chapter 4 · Concept 2 of 11

Conduction of Heat and Electricity

• Heat — Metals conduct heat well — that's why cooking vessels are metal, but their handles are wood (a poor conductor).
• Electricity — Aluminium, iron, and copper let a tester bulb glow. That's why electricians wear rubber gloves and use plastic-handled tools.

Chapter 4 · Concept 3 of 11

Metals That Break the Rules

• Soft metals — Sodium and potassium are soft enough to cut with a knife.
• A liquid metal — Mercury is a metal that stays liquid at room temperature.

Chapter 4 · Concept 4 of 11

What Causes Iron to Rust?

• An experiment with three sealed bottles shows the answer clearly.
• Dry air only — Nail + silica gel, sealed. No rust forms.
• Water only — Nail fully underwater, sealed with an oil layer. No rust forms.
• Air + water — Nail half in water, open to air. Rust forms.

Chapter 4 · Concept 5 of 11

Rust, Corrosion, and Prevention

• Rust and corrosion — Rust is the brown deposit on iron. Corrosion is the broader, gradual wearing away of any metal surface — like the green coating on copper or the black coating on silver.
• Preventing rust — Painting, oiling, greasing, or galvanisation (coating iron with zinc) all keep air and water away from the metal.

Chapter 4 · Concept 6 of 11

Burning Magnesium

• Burning magnesium ribbon produces a dazzling white flame and a white powder — magnesium oxide.
• Basic in nature — Magnesium oxide dissolved in water turns red litmus blue — metal oxides are generally basic.

Chapter 4 · Concept 7 of 11

Sodium: Stored in Kerosene

• Why kerosene? — Sodium reacts vigorously with both oxygen and water, releasing a great deal of heat — so it's kept away from air and water by storing it under kerosene.

Chapter 4 · Concept 8 of 11

Burning Sulfur

• Burning sulfur produces sulfur dioxide gas, which dissolves in water to turn blue litmus red.
• Acidic in nature — Non-metal oxides, unlike metal oxides, are acidic. Sulfur itself does not react with water at all.

Chapter 4 · Concept 9 of 11

Phosphorus: Stored in Water

• Why water? — Phosphorus catches fire the moment it's exposed to air, so it's stored underwater to keep air away from it.

Chapter 4 · Concept 10 of 11

Metals vs Non-metals

• Property | Metals | Non-metals
• Lustre — Shiny — Dull
• Malleability / Ductility — Yes — No — often brittle
• Sonority — Ring when struck — No ringing sound
• Conduction (heat, electricity) — Good — Poor
• Nature of oxide — Basic — Acidic

Chapter 4 · Concept 11 of 11

Non-metals Are Essential Too

• Oxygen — Essential for breathing and survival.
• Carbon — The building block of all life forms.
• Nitrogen — Used in fertilisers, essential for plant growth.
• Chlorine & Iodine — Chlorine purifies water; iodine is an antiseptic.

Quick Recap

Check Your Understanding

1. 1Which metal is commonly used for food packaging, since it's cheap and its thin sheets fold easily into any shape? (i) Aluminium (ii) Copper (iii) Iron (iv) Gold
2. 2Which metal catches fire when it comes into contact with water? (i) Copper (ii) Aluminium (iii) Zinc (iv) Sodium
3. 3Why are only a few metals suitable for making jewellery?